When to Use 50% as the Estimated Proportion
Many sample size calculators default to 50% when the expected proportion is unknown. This is not arbitrary. It is the conservative choice that protects you from underestimating the sample needed.
Why 50% is the conservative default
The variability term p x (1 - p) is largest when p = 0.5. That means the required sample size is largest at 50%, all else being equal.
Using 50% therefore gives you a safe estimate when you do not have prior information.
When to use a different value
If you have reliable prior data, you can use an estimated proportion that better reflects the expected result. For example, if past surveys show support is usually around 20%, using that value may give a more tailored estimate.
The key is that the assumption should be justified, not optimistic.
A practical rule
If you are unsure, use 50%. If you have credible prior evidence, use that prior estimate. That keeps your planning transparent and easier to explain.
Why this assumption is so common
Using 50% protects you against underestimating the sample when you have little or no prior evidence. It is a planning safeguard, not a claim that the actual result will land near fifty-fifty.
That is why this assumption is common in calculators, methods notes, and stakeholder discussions. It is easy to explain and difficult to misuse as long as people understand that it is conservative by design.
- Use 50% when you lack trustworthy prior data
- Replace it only when you can justify a better estimate
- Do not swap in a lower value just to shrink the sample
- Document why the chosen proportion is appropriate